This invention relates to vulcanizable diene rubber compounds which exhibit a high capacity for the addition of sulfur to be varied while processing safety is maintained, for the production of improved rubber moldings, particularly tire components.
Due to the dynamic stresses on a tire, tire components require good thermal and mechanical stability in order to ensure good durability of the tire. With regard to mechanical stability, a high modulus and a high resistance to tear propagation, in particular, are desirable at a given hardness of the vulcanized material.
The economics of the use of tires, particularly truck tires, is to a major extent determined by the total number of possible remolds of the tire tread. Repeated remolding of the tire tread is only possible, however, to the extent which is permitted by the stressed carcass of the tire.
xcex1,xcfx89-bis(N,Nxe2x80x2-diethylthiocarbamoyldithio)-alkanes and 1,2-bis(N,Nxe2x80x2-dibenzylthio-carbamoyldithio)-ethane are known from Patent Applications EP 385 072, EP 385 073, EP 432 405 and EP 432 417 as crosslinking agents for diene rubber for the production of tire treads and tire sidewalls. A common feature of diethylamine derivatives is that they can release diethylamine during the vulcanization of the rubber compound. Diethylamine can form diethylnitrosamine, which is carcinogenic to humans. 1,2-bis-(N,Nxe2x80x2-dibenzylthiocarbamoyldithio)-ethane is not, in fact, capable of forming dibenzylnitrosamine, which is carcinogenic to humans (see Druckrey et al., Z. Krebsforschung 69 (1967) 103), but based on the teaching of DE 22 56 511 the possibility cannot be ruled out for this compound, with its 1,2-dithioethanediyl radical, resulting in malodorous vulcanized materials (see page 17, paragraph 2), whereas xcex1,xcfx89-dithioalkanediyl radicals which comprise more than 4 carbon atoms result in vulcanized materials which result in no problems as with respect to odor.
In the aforementioned European Patent Applications, the crosslinking agents were used without sulfur additives. It is mentioned that the additional use of sulfur generally results in no advantages, since the reversion behavior of the vulcanized materials deteriorates.
EP 530 590 discloses a method of producing vulcanized diene rubber materials with a crosslinking agent system consisting of 1 to 4.5 parts by weight of 1,2-bis(N,Nxe2x80x2-dibenzylthiocarbamoyldithio)-ethane or of 1,6-bis(N,Nxe2x80x2-dibenzylthiocarbamoyldithio)-hexane, 0.05 to 0.3 parts by weight sulfur, and selected amounts of vulcanization accelerators. The particular teaching of the aforementioned patent is that the amount of crosslinking agent used can be reduced (economic aspects) by the use of very small amounts of sulfur, and that elastomers which exhibit a resistance to aging and reversion which was unattainable can be obtained with the avoidance of bloom phenomena and after relatively short times of vulcanization (page 7, lines 25 to 27). Vehicle tires and engine mountings are cited as examples of the use of vulcanized materials such as these.
Furthermore, DE 22 56 511 contains a very general discussion of the use of compounds of general formula A xe2x80x94Sxe2x80x94Sxe2x80x94Rxe2x80x94Sxe2x80x94Sxe2x80x94Axe2x80x2for the vulcanization of rubbers, wherein R constitutes almost any divalent organic radical, and A and Axe2x80x2 constitute a very large number of accelerator radicals, which also include N-substituted thiocarbamoyl radicals amongst others. The crosslinking agents can be used on their own or can also be used in combination with sulfur and vulcanization accelerators. The addition of elemental sulfur preferably falls within the range from 0.5 to 1.5 parts by weight sulfur with respect to 100 parts by weight rubber. Page 32, paragraph 2 contains the teaching that the crosslinking agents of the invention, in combination with amounts of sulfur greater than 1.5 parts by weight, result in a decrease in the level of processing safety of these rubber compounds.
One measure of the level of processing safety is the Mooney scorch time. A short Mooney scorch time signifies a low level of processing safety, whereas a long Mooney scorch time signifies a high level of processing safety. A high level of processing safety of rubber compounds is very desirable, because the addition of vulcanization retarders can thereby be dispensed with.
It is shown in DE 22 56 511, using master batch A (an NR compound), that the rubber compound according to Example 7 of Table VII, which comprises 1.0 parts by weight 1,2-bis-N,Nxe2x80x2-dimethylthio-carbamoyldithio)-ethane and 1.7 parts by weight sulfur, has a Mooney scorch time (t5/121xc2x0 C.) of only 25.7 minutes. Compared with this, the compound according to Example 1 of Table III (control example), which comprises 2.0 parts by weight sulfur and 0.5 parts by weight Santocure NS (benzothiazyl-2-tert.-butylsulphenamide, TBBS) as an accelerator, has a corresponding Mooney scorch time of 32.0 minutes, i.e. the level of processing safety of the rubber compound in Example 7 of Table VII, which comprises 1.7 parts by weight sulfur, has actually become inferior to that of the control example. As evidenced by Example 6 in Table VII, there is in fact a deterioration of the level of processing safety even at an addition of sulfur of more than 1.0 part by weight, as measured on the control compound (see Example 1, Table III). Rubber compounds which contain the crosslinking agents of the invention are suitable for the bonding or agglutination of natural or synthetic textile fibers (page 35, paragraph 2).
As described at the outset, there is, therefore, a pressing need for vulcanizable rubber compounds for the production of improved tire components, particularly with respect to industrial hygiene, processing safety of the rubber compound, and the mechanical and dynamic property profiles and aging behavior of vulcanized materials.
The object of the present invention is to provide vulcanizable rubber compounds, which exhibit a high capacity for the addition of sulfur to be varied while maintaining high processing safety, for the production of improved tire components. The rubber compounds according to the invention release no nitrosamines during vulcanization which are carcinogenic to humans. The vulcanized materials resulting from the rubber compounds are free from undesirable odors of organosulfur compounds, and not only do the vulcanized materials exhibit no deterioration, or exhibit only a slight deterioration in their technological properties during aging, but they are even improved before aging with respect to modulus and resistance to tear propagation, or their modulus remains virtually of the same standard while their resistance to tear propagation is improved without any deterioration in the standard of their other important technological properties, such as the loss factor tan xcex4 at 70xc2x0 C. or the heat build-up.
In this connection, it should be mentioned that the modulus and the resistance to tear propagation are usually diametrically opposed to each other, i.e. vulcanized materials with a high modulus usually exhibit a low resistance to tear propagation, and vice versa.
The object of the present invention has surprisingly been achieved by the vulcanization of a rubber compound based on diene rubbers which comprise a special crosslinking agent which provides C6-bridges, in combination with a selected amount of sulfur, and in the presence of vulcanization accelerators.
The present invention therefore relates to vulcanizable rubber compounds based on diene rubbers and customary additives, which are characterized in that the vulcanizing system contained in the compounds comprises
a) 0.5 to 3.8 parts by weight of compound (I)
R2Nxe2x80x94(Cxe2x95x90S)xe2x80x94Sxe2x80x94Sxe2x80x94(CH2)xxe2x80x94Sxe2x80x94Sxe2x80x94(Cxe2x95x90S)xe2x80x94NR2xe2x80x83xe2x80x83(I) 
where R=(C6H5CH2);
and X=6,
b) 0.5 to 2 parts by weight sulfur and
c) 0.5 to 3.0 parts by weight of vulcanization accelerators, wherein the parts by weight are given in each case with respect to the use of 100 parts by weight of rubber.